In the metallization step utilized in the production of integrated circuits preparatory to etching of conductors and bonding pads about their outer surfaces, step-coverage of conductive metal films (typically low boiling point metals, such as aluminum and/or copper alloys) is poor over surface discontinuities, such as via holes, where contact bonding must take place.
Step coverage of conductive metal films deposited conventionally by evaporation or sputtering becomes progressively worse as the dimensions of components on the integrated circuit shrink. The poor step coverage is a result of the "shadow effect" in the deposited film at the sidewalls of steps or holes.
Although the above-mentioned step coverage problem can be solved to some degree by either chemical vapor deposition of tungsten, or by metal deposition using high temperature and/or bias sputtering, the improvement in the step coverage is achieved at the expense of several drawbacks. For chemical vapor deposition of tungsten, the film resistivity is about three times higher than that of aluminum of aluminum alloys. On the other hand, high temperature and/or bias sputtering usually results in poor film qualities such as low electron migration resistance, and high dislocation density.
The use of a pulsed laser to melt and planarize thin metal films having low boiling points to fill high aspect ratio contact vias is a very attractive approach to current high density circuit metallization. Planarization of the conductive surface is particularly desirable when vias are stacked vertically in multilevel metallization. Laser planarization is a low thermal budget, simple, and effective technique for planarizing conductive metal layers and filling interlevel contacts at the cost of only one additional step to the standard process flow.
Excimer laser planarization relies on a very short laser pulse to rapidly melt an absorbing metal layer. During the molten period, mass transport of the conductive metal occurs, which results in flow of the metal into vias and drives the surface flat due to the high surface tension and low viscosity of molten metals.
Recently, the technique of laser planarization has shown promise in improving the step coverage of aluminum alloy films in micron/submicron geometry contacts and contact vias. However, due to the high reflectivity of aluminum (approximately 93% for wave lengths in the region down to 200 mm) and its relatively low evaporation temperature (2467.degree. C.), aluminum alloys suffer from the following disadvantages: (1) inefficient use of laser energy, (2) low optical ablation limit, and (3) low process window between the ablation limit and the via-fill limit.
Planarization systems utilizing excimer laser irradiation show particular promise for filling submicron-diameter vias and planarizing the resulting surface. Lessening of the surface reflectivity normally encountered in heating of aluminum alloys by laser energy has already been reported as widening the "process window" between the "ablation limit", or temperature at which the conductive metal boils or evaporates, and the "via-fill limit", or temperature at which sufficient flow of the conductive metal occurs to fill the circuit recesses.
A general discussion of laser planarization can be found in a paper titled "Interconnects on Integrated Circuits Improved by Excimer Laser Planarization for Multilevel Metallization" by Mukai, et al., pp. 101-107, i.e., VLSI Multilevel Interconnection Conference, Santa Clara, CA (1988), which is hereby incorporated into this disclosure by reference. It describes the use of a thin copper overcoating to enhance aluminum planarization processing by increasing the initial optical absorbance of the laser beam in the conductive metal film. The paper fails to address the generally recognized low oxidation resistance of copper and the difficulty of subsequently etching such copper coatings.
Use of titanium as an anti-reflective coating for laser planarization processes has also been proposed. However, reported improvements in planarization were achieved at the expense of several drawbacks, including high resistivity and stress. The higher resistivity of the Ti--Al alloys that result from intermixing of these materials during laser planarization diminishes the advantage of aluminum metallization over alternative metallization schemes using chemical vapor deposited tungsten as the primary conductive medium. Moreover, the higher stresses in the resulting Ti--Al alloys imposes reliability concerns, such as adhesion, cracks and stress voiding. It has therefore been concluded by prior researchers that titanium itself is not a desirable anti-reflective coating for aluminum and aluminum alloys in metallization procedures.
Despite the shortcomings in presently-reported systems for laser planarization, the value of an anti-reflective coating in widening the process window has been concluded to be important and to have demonstrated usefulness in increasing the thickness of a layer of conductive metal across a step or via.
A search for alternative anti-reflective coatings has led to the present identification of deposited layers of high boiling point metals, such as tungsten or an alloy of tungsten and titanium, as a useful coating. A layer of the selected metal is proposed as an anti-reflective coating on aluminum alloys or other low boiling point metals used for metallization purposes. The addition of a the metallic film prior to laser planarization results in more efficient use of laser energy, less ablation of the aluminum layer at a given optical fluence, and widening of the process window. Its application over the conductive metal film is controlled to eliminate or minimize intermixing of the anti-reflective coating and metallization layer during laser planarization. The anti-reflective layer can then be substantially removed by etching, leaving the metallization layer exposed for further conventional processing steps.